Digging Deeper: The influence of historic mining on Glasgow's subsurface thermal state to inform geothermal research

Studies of the former northeast England coalfield in Tyneside demonstrated heat flow perturbations in boreholes to be due to the entrainment and lateral dispersion of heat from deeper in the subsurface through flooded mine workings. This work assesses the influence of historic mining on geothermal observations across Greater Glasgow. The regional heat flow for Glasgow is 60 mW m^-2 and after correction for palaeoclimate is estimated as ~80 mW m^-2. An example of reduced heat flow above mine workings is observed at Hallside (~10 km SE of Glasgow), where the heat flow through a 352 m deep borehole is ~14 mW m^-2. Similarly, the heat flow across the 199 m deep GGC01 borehole in the Glasgow Geothermal Energy Research Field Site is ~44 mW m^-2. The differences between these values and the expected regional heat flow suggests a significant component of horizontal heat flow into surrounding flooded mine workings. This deduction also influences the quantification of deeper geothermal resources, as extrapolation of the temperature gradient above mine workings would underestimate the temperature at depth. Future projects should consider the influence of historic mining on heat flow when temperature datasets such as these are used in the design of geothermal developments.